A vehicle seat is generally equipped with a seat belt assembly for restraining the vehicle occupant for preventing the vehicle occupant from being thrown forward in case of a vehicle crash. A seat belt assembly normally includes a retractor assembly incorporated with an emergency locking device (ELR device) which normally does not restrain the vehicle occupant but locks up the winding shaft of the seat belt only at the time of a sudden stop or a vehicle crash. An ELR device can lock up the winding shaft in an extremely short period of time after detecting a deceleration exceeding a prescribed threshold level, and prevent the seat belt from being paid out any further.
However, simply locking up the winding shaft is not sufficient for preventing a small length of the seat belt to be paid out because of an inevitable amount of slack or looseness of the part of the seat belt wound around the winding shaft. In view of this fact, a device called "pre-tensioner" has been proposed to increase the restraining capability of the seat belt by positively pulling the seat belt buckle to which the seat belt is attached at the time of a vehicle crash (see U.S. Pat. No. 4,705,296). In such pre-tensioners, the use of high pressure gas generated from a chemical reaction, and a resilient force of a spring has been customary.
When a piston is actuated by chemically produced high pressure gas, and the resulting thrust is transmitted to the buckle via a length of cable, a relatively high tension is applied to the cable, and the tensile strength of the cable has to be accordingly great. However, cable having a high tensile strength tends to have a high rigidity, and therefore tends to oppose a force to bend it. It means an increase in the resistance in pulling the cable along a tortuous path, and an increase in the necessary amount of the chemical agent for producing the gas. This is not desirable not only from the view point of cost but also because of a large amount of combustion gas which is produced from the combustion of the chemical agent, and is known to be highly unpleasant for the vehicle occupant. When the resilient force of a spring member is employed for pulling the seat-belt buckle, in order to meet a speed requirement, the spring member has to have a high rigidity, and tends to be large in size. Also, the assembly work tends to be troublesome.
Also, to avoid yielding of the seat belt or the buckle once it has been rapidly pulled, a pre-tensioner device is typically incorporated with a reversal preventing device such as a ratchet mechanism or a one-way clutch in addition to an ELR device. However, although it is essential to restrain the vehicle occupant from being thrown forward by the impact of a vehicle crash, it is desired that the impact acting upon the vehicle occupant may be no more than the level which the vehicle occupant is capable of safely taking because in case of a relatively large crash impact, if the seat belt or the buckle is firmly locked up after it has been rapidly pulled, the full force of the impact of the crash is applied to the vehicle occupant with the exception of a certain amount of impact absorbed by the slight elongation of the seat belt itself. Similarly, a seat recliner assembly is normally adapted to firmly lock up at selected positions, but it may be desirable if the assembly is capable of absorbing some of the impact of a vehicle crash by increasing the reclining angle at a controlled rate while absorbing the crash impact.